Electrochemical Approaches for the Synthesis and Characterization of Innovative Electrode Materials

The synthesis of nanoscale catalysts is a cornerstone technology to meet the current energy challenges, spanning in a wide range of applications from fuel cell efficiency to pollutant degradation [1]. On this regard, electrocatalysis is a field of particular interest and one of the most promising, owing to the ability of electrochemical methods to study the reaction mechanisms and electron transfer (ET) dynamics at a molecular level, at the electrode surface. With the rapid advances in nanostructure synthesis, new nanoscale catalysts and novel catalytic properties continue to emerge [2]. Often, nanoscale catalysts are used on electrodes, where the electrode modifies the kinetics of the process reducing the overpotential, hence performing electrocatalysis. The development and use of new electrodic materials and modified electrodes in the field of applied electrochemistry, energy and organic electrosynthesis is today an important way to obtain catalysts with controlled structure and morphology, and reactions with high selectivity. Control over a variety of physical and chemical phenomena ranging from catalysis to electron transfer is often achieved through the extensive use of modified surfaces. There are basically two possibilities for the modification of an electrode surface, (i) to alter its structure or (ii) to modifiy its composition (or both) [3]. The emergence of fuel cell technology has created a new tool for the generation of new catalysts to replace the expensive and rare platinum, which contributes to over 55% of the total costs. Reducing the Pt loading (particularly in the cathode catalyst layer) without compromising the fuel cell performance is an effective strategy to meet the cost requirements for its commercialization. In addition, the design of novel electrocatalysts requires not only reducing the amount of Pt used, but also enhancing catalytic activity and durability. In fuel cells (FCs), the oxygen reduction reaction (ORR) is the reaction occurring at the cathode. ORR in a typical PEM fuel cell is a slow reaction; using the state-of art platinum catalysts the overpotential of ORR is found to be 300 mV higher than that of the oxidation reaction at the anode [4]. Ideally, the standard cell potential, E0cell, is 1.23 V with a negative Gibbs free energy, implying a spontaneous reaction that can be utilised as an electrical energy generator. Practically, however, the cell potential is less than 1.23 V due to an overpotential in the cell, primarily related to the oxygen reduction reaction at the cathode, which is kinetically sluggish because a large amount of energy is needed to break the strong oxygen-oxygen double bond during the process. For example, Nitrogen-doped carbon and its composites possess a great potential for fuel cell catalyst applications, especially at the oxygen reduction reaction. The ORR mechanisms of nitrogen-doped carbon catalysts seem to involve adsorption of oxygen [5] at the partially polarised carbon atoms adjacent to the nitrogen dopants, in contrast to the mechanism at metal catalysts (i.e. Pt, Pd), which utilise d-bands to form bonds with oxygen. Indeed, in the field of organic electrocatalysis, a topic of great interest is the behaviour of catalytic electrode materials for the reduction of organic halides [6]. In fact, the choice of electrode materials is often crucial for the success of electrochemical reactions in order to achieve selective reactions. In our own experience, some electrode materials such as Ag, Cu or Pd in organic solvents (MeCN, DMF, etc.) showed extraordinary catalytic properties for the electroreduction of organic halides [7]. This allows not only the reduction of the substrates at more positive potentials, but also makes possible modification of the reaction mechanism and selectivity [8]. It is thus of great interest to find new catalytic materials to create more favourable conditions for electrochemical processes. Therefore, it is of primary importance to characterize decisive parameters influencing the reactivity and the selectivity of a catalytic surface, in order to use this knowledge for the systematic design of better catalysts. The parameters influencing the electrocatalytic properties of nanoparticles [9,10] can be: (i) particle size, (ii) particle dispersion, (iii) density of low-coordinated surface atoms and (iv) effect of the substrate material. In order to evaluate these parameters, model surfaces without chemical and morphological defects (HOPG) and monolayer nanostructures deposited on them must be investigated. Usually, the modified electrodes are built from an inert support, such as graphite (HOPG), glassy carbon (GC) or mesoporous carbon (MC), on which metal nanoparticles are deposited. These supports are very interesting because, like all carbon-based supports, they have chemical stability, wide potential window and low background current. This PhD thesis project intends to prepare and characterize different carbon materials, before and after doping with heteroatoms (particularly Nitrogen), with aim of investigating their catalytic properties in the ORR and in the reduction of organic halides. After that, we will try to deposit metal nanoparticles prepared by evaporation in ultra-high-vacuum (UHV), or by electrochemical and chemical means, to compare the results achieved using the three deposition methods and gain a better indication about the possibility to device a real electro-catalyst in the different fields described above. The first task includes the research for the best experimental conditions to obtain the new electrode materials. To this end, UHV conditions will be investigated to synthetize N-HOPG and N-GC, whereas chemical synthesis followed by annealing will be used to prepare N-MC. After evaluation of the chemical and morphological properties of the materials, the second goal is to investigate their catalytic properties toward the electrochemical reduction reactions of oxygen and organic halides, reactions that can be influenced by solvent, pH, supporting electrolyte, working potential and electron transfer mechanism. The physical and chemical characterizations will be carried out using Cyclic Voltammetry (CV), Rotating Disk Electrode (RDE), X-ray Photoelectron Spectroscopy (XPS), Transmission and Scanning Electron Microscopy (TEM, SEM) and Brunauer-Emmett-Teller theory (BET)